Many radio receivers and amplifiers have an AGC feature for reducing the amplification of one or more gain stages to compensate for the varying levels of signal intensity that may be received. Such AGC circuits permit modern receivers to have dynamic ranges in excess of 100 dB.
In certain applications, particularly at high frequencies and low signal strengths, the noise figure of the receiver becomes an important consideration. The receiver noise figure is most directly affected by the gain of the receiver front end. Consequently, it is desirable to operate the front ends of sensitive receivers at maximum gains and to gain control subsequent stages. Only after the received signal strength is so large that noise figure is not an important issue should gain reduction occur in the front end stage.
Prior art systems typically address the noise figure problem by running the front end stage without any gain control whatsoever. While optimizing the noise figure, such an approach degrades the overall system dynamic range since signals above a certain threshold overwhelm the receiver. Thus, there is a continuing need for improved AGC systems that will progressively reduce the gain of the amplifier stages, beginning with those stages most remote from the front end.
Accordingly, it is an object of the present invention to provide an improved AGC system for receivers which will progressively reduce the gain in the amplifier stages, beginning first with the stages most remote from the front end.
It is a further object of the present invention to provide such an improved AGC system with a high degree of noise immunity.
It is still a further object of the present invention to provide such an improved AGC system that can be fully implemented using integrated circuit fabrication techniques.
According to the present invention, the staggering of the onset of gain reduction in a series of cascaded gain stages is effected by controlling the area ratio between corresponding components in two or more AGC control circuits whose topologies are otherwise similar. A representative AGC control circuit may include two transistors in a differential common-emitter configuration. The bases of the transistors are driven from a signal related to the amplitude of signal being received by the receiver. The collectors of the transistors provide a differential AGC output signal controllably offset from the input signal by a voltage determined by the area ratio between the two transistors. The differential configuration is preferably employed to reject common mode signals and thus to improve noise immunity. Several such AGC circuits can be employed to cause different amplifier stages in the receiver to begin significant gain reduction at different received signal thresholds.
The foregoing and other objects, features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.